1. Field of the Invention
This invention relates to the field of signal processing. More particularly, this invention relates to processing measured signals to remove unwanted signal components caused by noise and especially noise caused by motion artifacts. Even more particularly, the invention relates to a method, apparatus and system for removing motion artifacts in the context of a pulse oximeter.
2. Description of the Related Art
The measurement of physiological signals can often be difficult because the underlying physiological processes may generate very low level signals. Furthermore, interfering noise is inherent in the body and the interface between the body and sensors of physiological processes. Examples of physiological measurements include: measurement of electrocardiogram (ECG) signals based on the electrical depolarization of the heart muscle, blood pressure, blood oxygen saturation, partial pressure of CO2, heart rate, respiration rate, and depth of anesthesia. ECG signals, for example, are typically detected by surface electrodes mounted on the chest of a patient. ECG signals are weak at the signal source (i.e., the heart) and are even weaker at the surface of the chest. Furthermore, electrical interference from the activity of other muscles (e.g., noise caused by patient breathing, general movement, etc.) causes additional interference with physiological signals such as an ECG. Thus, considerable care must be taken in the design and use of physiological processors to enhance the quality of the true signal and reduce the effects of interfering noise signals.
It is convenient to characterize a measured signal as being a composite signal composed of a true signal component and a noise signal component. The terms “measured signal” and “composite signal” will be used interchangeably hereinafter. Signal processors are frequently used to remove noise signal components from a composite measured signal in order to obtain a signal that closely, if not identically, represents the true signal. Conventional filtering techniques such as low pass, band pass, and high pass filtering can be used to remove noise signal components from the measured composite signal where the noise signal component occupies a frequency range outside the true signal component. More sophisticated techniques for conventional noise filtering include multiple notch filters, which are suitable for use where the noise signal component exists at multiple, distinct frequencies, all outside the true signal frequency band.
However, it is often the case that the frequency spectrum of the true and noise signal components overlap and that the statistical properties of both signal components change with time. More importantly, there are many cases where little is known about the noise signal component. In such cases, conventional filtering techniques may be ineffective in extracting the true signal.
The measurement of oxygen saturation in the blood of a patient is a common physiological measurement, the accuracy of which may be compromised by the presence of noise. Knowledge of blood oxygen saturation can be critical during surgery. There are means of obtaining blood oxygen saturation by invasive techniques, such as extracting and testing blood removed from a patient using a co-oximeter. But, such invasive means are typically time consuming, expensive, and uncomfortable for the patient. Fortunately, non-invasive measurements of blood oxygen saturation can be made using known properties of energy attenuation as a selected form of energy passes through a bodily medium. Such non-invasive measurements are performed routinely with a pulse oximeter.
The basic idea behind energy attenuation measurements as employed in pulse oximetry is as follows. Radiant energy is directed toward a bodily medium, where the medium is derived from or contained within a patient, and the amplitude of the energy transmitted through or reflected from the medium is then measured. The amount of attenuation of the incident energy caused by the medium is strongly dependent on the thickness and composition of the medium through which the energy must pass, as well as the specific form of energy selected. Information about a physiological system can be derived from data taken from the attenuated signal of the incident energy transmitted or reflected. However, the accuracy of such information is reduced where the measured signal includes noise. Furthermore, non-invasive measurements often do not afford the opportunity to selectively observe the interference causing the noise signal component, making it difficult to remove.
A pulse oximeter is one example of a physiological monitoring system that is based upon the measurement of energy attenuated by biological tissues and substances. More specifically, a pulse oximeter measures the variable absorption caused by blood volume changes, primarily arterial in origin. Pulse oximeters transmit electromagnetic energy at two different wavelengths, for example at 660 nm (red) and 940 nm (infrared, hereinafter IR) into the tissue and measure the attenuation of the energy as a function of time. The output signal of a pulse oximeter is sensitive to the pulsatile portion of the arterial blood flow and contains a component that is a waveform representative of the patient's arterial pulse. This type of signal, which contains a component related to the patient's pulse, is called a plethysmographic waveform or plethysmogram.
The period of rhythmic contraction of the heart by which blood is driven through the aorta and pulmonary artery is known as systole. Maximum light absorbance occurs during the systole of a cardiac cycle and is indicated on a plethysmogram by a low point or systolic valley. Conversely, the period of rhythmic relaxation and dilation of the heart cavities occurs during diastole when blood is drawn into the heart cavities. Minimum light absorbance occurs during the diastole of a cardiac cycle and is indicated on a plethysmogram by a high point or diastolic peak.
Pulse oximetry measurements typically use a digit, such as a finger, or an ear lobe or other element of the body, where blood flows close to the skin as the medium through which light energy is transmitted. The finger, for example, is composed of various tissues and substances including skin, fat, bone, muscle, blood, etc. The extent to which each of these biological tissues and substances attenuate incident electromagnetic energy is generally known. However, the effect of motion can cause changes in the optical coupling of the sensor (or probe) to the finger, the underlying physiology, the local vasculature, optical properties of tissues due to changing optical path length as well as combinations and interactions of all of the above. Thus, patient motion may cause erratic energy attenuation.
A typical pulse oximeter includes a sensor, cabling from the sensor to a computer for signal processing and visual display, the computer and visual display typically being included in a patient monitor. The sensor typically includes two light emitting diodes (LEDs) placed across a finger tip and a photodetector on the side opposite the LEDs. The detector measures both transmitted light signals once they have passed through the finger. The signals are routed to a computer for analysis and display of the various parameters measured.
The underlying physical basis of a pulse oximeter is Beer's law (also referred to as Beer-Lambert's or Bouguer's law) that describes attenuation of monochromatic light traveling through a uniform medium that absorbs light with the equation:Itransmitted=Iincident·e−dcα(λ),  (1)
where Itransmitted is the intensity of the light transmitted through the uniform medium, Iincident, is the intensity of incident light, d is the distance light is transmitted through the uniform medium, c is the concentration of the absorbing substance in the uniform medium, expressed in units of mmol L−1, and α(λ) is the extinction or absorption coefficient of the absorbing substance at wavelength λ, expressed in units of L/(mmol cm). The properties of Beer's law are valid even if more than one substance absorbs light in the medium. Each light absorbing substance contributes its part to the total absorbance. However, Beer's law does not strictly apply since an LED's output is not monochromatic and scattering effects do have a significant influence. Thus, manufacturers often utilize an empirically determined lookup table to map from the ratio of absorbance (or transmittance) at the red and IR frequencies to a saturation value.
Two LEDs emit narrowband light (i.e., half power bandwidth of typically 15 nm) at two different frequency bands, typically red (centered at about 660 nm) and IR (centered at about 940 nm). The intensity of light transmitted through tissue, Itransmitted, is different for each wavelength of light emitted by the LEDs. Oxyhemoglobin (oxygenated blood) tends to absorb IR light, whereas deoxyhemoglobin (deoxygenated blood) tends to absorb red light. Thus, the absorption of IR light relative to the red light increases with oxyhemoglobin. The ratio of the absorption coefficients can be used to determine the oxygen saturation of the blood.
To estimate pulsatile blood oxygen saturation, SpO2, a two-solute concentration is assumed. A measure of functional blood oxygen saturation level, SpO2, can be defined as:
                                                        Sp              ⁢              O                        2                    =                      100            ·                                          c                0                                                              c                  r                                +                                  c                  0                                                                    ,                            (        2        )            where co represents oxyhemoglobin solute concentration, and c, represents reduced or deoxyhemoglobin solute concentration.
Noise signal components in a measured pulse oximetry light signal can originate from both AC and DC sources. DC noise signal components may be caused by transmission of electromagnetic energy through tissues of relatively constant thickness within the body, e.g., bone, muscle, skin, blood, etc. Such DC noise signal components may be easily removed with conventional filtering techniques. AC noise signal components may occur when tissues being measured are perturbed and, thus, change in thickness while a measurement is being made. Such AC noise signal components are difficult to remove with conventional filtering techniques. Since most materials in and derived from the body are easily compressed, the thickness of such matter changes if the patient moves during a non-invasive physiological measurement. Thus, patient movement can cause the properties of energy attenuation to vary erratically. The erratic or unpredictable nature of motion artifacts induced by noise signal components is a major obstacle in removing them.
Various approaches to removing motion artifacts from measured physiological signals, and particularly for use in pulse oximeters, have been proposed. U.S. Pat. Nos. 5,482,036; 5,490,505; 5,632,272; 5,685,299; 5,769,785; and 6,036,642, all to Diab et al., and U.S. Pat. No. 5,919,134 to Diab, disclose methods and apparatuses for removing motion artifacts using adaptive noise cancellation techniques. The basic proposition behind these Diab et al. patents is to first generate a noise reference signal from the two measured signals, and then use the noise reference signal as an input to an adaptive noise canceller along with either or both of the measured signals to remove the reference noise signal from the measured signals, thus approximating the actual parametric signals of interest. These Diab et al. patents appear to require the use of both measured input signals to generate a noise reference signal. Where the adaptive noise cancellation involves the use of a correlation canceller as disclosed in U.S. Pat. No. 5,482,036, additional problems include significant computational overhead and under certain circumstances, the correlation canceller will drive the output signal to zero.
Another approach to noise artifact elimination is disclosed in U.S. Pat. No. 5,588,427 to Tien. Tien uses fractal dimension analysis to determine the complexity of waveforms in order to determine the proper value of the ratio of true intensities based on signal complexity. The Tien approach employs a fractal analyzer to determine values for two ratios, α and β, based on the measured time varying intensity of the transmitted red and IR light signals including noise. α is defined as the ratio of the time varying true intensity of light transmitted from the red LED and the time varying true intensity of the light transmitted from the IR LED. β is a similar ratio relating the noise introduced during the measurement of the light transmitted by the red LED and the noise introduced during the measurement of the light transmitted by the IR LED. According to Tien, a fractal analyzer then determines values for α and β and provides (α,β) pairs to a statistical analyzer. The statistical analyzer performs analysis of one or more (α,β) pairs to determine the best value for α, which is then provided to a look-up table. The look-up table provides a value corresponding to the arterial oxygen saturation in the patient. While the Tien approach appears to be an innovative use of fractal analysis, it also appears to be computationally complex.
Yet another approach to noise artifact elimination is disclosed in U.S. Pat. Nos. 5,885,213; 5,713,355; 5,555,882; and 5,368,224, all to Richardson et al. The basic proposition behind the Richardson et al. approach is to switch operative frequencies periodically based on evaluating the noise level associated with various possible frequencies of operation in order to select the frequency of operation that has the lowest associated noise level. It would appear that data measured at a noisy frequency, using the Richardson et al. approach could be invalid or useless for calculating arterial oxygen saturation. Furthermore, Richardson et al. requires a computational overhead to constantly monitor which frequency of operation provides the least noise.
Another approach to noise artifact elimination is disclosed in U.S. Pat. No. 5,853,364 to Baker, Jr. et al. The Baker, Jr. et al. approach first calculates the heart rate of the patient using an adaptive comb filter, power spectrum and pattern matching. Once the heart rate is determined, the oximetry data is adaptively comb filtered so that only energy at integer multiples of the heart rate are processed. The comb filtered data and the raw oximetry data are filtered using a Kalman filter to adaptively modify averaging weights and averaging times to attenuate motion artifact noise. The adaptive filtering of the Baker, Jr. et al. approach appears to add significant computational complexity to solve the problem of motion artifact rejection.
Still another approach to noise artifact elimination is disclosed in U.S. Pat. No. 5,431,170 to Mathews. Mathews couples a conventional pulse oximeter light transmitter and receiver with a transducer responsive to movement or vibration of the body. The transducer provides an electrical signal varying according to the body movements or vibrations, which is relatively independent of the blood or other fluid flow pulsations. Mathews then provides means for comparing the light signals measured with the transducer output and performing adaptive noise cancellation. An apparent disadvantage of the Mathews approach is the need for a secondary sensor to detect motion.
Still yet another approach to noise artifact elimination is disclosed in U.S. Pat. No. 6,002,952 to Diab et al (hereinafter the '952 patent). Diab et al. recognizes the limitations of adaptive noise cancellation and particularly the use of a correlation canceller. The '952 patent discloses the use of frequency domain analysis to extract a pulse rate from oximetry data. According to the '952 patent, coupling coefficients related to ratios of uncontaminated measurement data and contaminated (noisy) measurement data can be determined from taking the ratios at each of a series of spectral peaks identified in the frequency domain.
The '952 patent further discloses using the coupling coefficients to identify the presence of noise by calculating the difference between the largest and smallest ratio lines for all spectral peaks, determining whether that difference is greater than a pre-selected threshold and whether the frequencies associated with the largest and smallest spectral peaks are arbitrarily close or not to each other. Where noise is detected, a scale factor is used to scrub the measurement data by controlling the gain control input of a gain controlled amplifier. The scale factor is zero in the presence of no noise, and can range up to the largest ratio line where there is noise and the frequencies are not close together. However, the signal scrubbing disclosed in the '952 patent appears to rely on a very limited measure of noise, i.e., whether the difference between the largest and smallest ratio lines is greater than a pre-selected threshold and how close the associated frequencies of largest spectral peak and the smallest spectral peak are relative to one another. It would be preferable to have multiple confidence measures in a method or system for determining physiological parameters in the presence of motion artifacts, e.g., a robust pulse oximeter.
Thus, a need in the art exists for a method, apparatus and system to eliminate motion-induced noise artifacts from light signals, that is relatively simple computationally, and that does not require more than one sensor, does not use correlation cancellers or adaptive noise cancellation and that uses multiple measures of confidence to determine physiological parameters accurately.